The present invention is concerned with a continuous extrusion machine, and a method of operation for continuously extruding non-ferrous metals such as aluminium and copper.
In general a continuous extrusion machine comprises a chassis a wheel and tooling. The tooling consists principally of a shoe and a die. The chassis supports the wheel for rotation by a motor. An endless groove is formed in the periphery of the wheel into which is entrained a feedstock which is commonly a bar of a non-ferrous metal such as aluminium or copper but may comprise metal particles or molten metal. Part of the periphery of the wheel is closely enveloped by the shoe so that the groove cooperates with the shoe to form a passage in use feedstock entrained in the groove enters the passage at an open end as the wheel rotates. The other end of the passage is obstructed by an abutment which is mounted on the shoe and intrudes into the passage. Because the feedstock is confined in the passage and the wheel continues to rotate, the feedstock is heated by friction with the groove. A die is mounted in a chamber formed in the shoe immediately upstream of the abutment. Eventually the thermal and other stresses imposed on the feedstock cause the feedstock to extrude through the die.
The continuous extrusion machine is capable of continuously extruding a wide range of sections of non-ferrous metal, for so long as feedstock is delivered to the groove.
In order to operate successfully it is necessary to have a small gap between the periphery of the wheel and the shoe. This gap permits a small quantity of the feedstock known as the flash, to extrude out of the passage onto the periphery of the wheel and into the gap. The size of the gap has a significant effect on the performance of the machine in terms of the speed, quality and type of extrusion which can be produced. Conventionally the gap is set before starting the machine. However, when the machine is in operation heat causes thermal expansion of the machine components and pressure on the wheel and chassis causes elastic deformation so that the gap size changes.
Thermal expansion typically alters the gap by up to 0.7 mm while elastic deformation alters the gap by between 0.3 and 0.5 mm. The effects of thermal expansion and extrusion pressures are non-uniform, will vary during start up, and may vary during operation and conventionally cannot be measured accurately.
The elastic deformation is relieved when feedstock ceases to enter the machine, as at shut down, and it is essential that the shoe does not impinge on the wheel or serious damage will occur. It is consequently not possible to pre-set the machine to run with a gap of less than the elastic deformation. It is also disadvantageous that the gap cannot be varied and accurately measured during machine operation in order to test the performance of various clearances in the production of an extrusion.
Accordingly the present invention provides a continuous extrusion machine having a chassis supporting a wheel for rotation and a shoe enveloping a span of the periphery of the wheel and co-operating with a groove formed in the periphery of the wheel to form a passage, a support mechanism supporting said shoe and/or wheel to be relatively displaceable in a direction perpendicular to the axis of rotation of the wheel during use, a gap sensor system able to sense the size of a gap between the wheel periphery and the shoe when the machine is operating, and control means responsive to the gap sensor to adjust the support mechanism to displace the shoe relative to the wheel.
The gap sensor system may also sense the shape of the gap.
In practice it is preferable to support the shoe via the support mechanism. However, the fundamental objective is to be able to accurately control the gap size and shape and so the displacement of the wheel relative to the chassis is deemed within the broad concept of this invention. Also within the scope of this invention is the displacement of the shoe and the wheel relative to the chassis particularly where it may be convenient to displace the shoe on one axis and the wheel on another.
A preferred support mechanism comprises a hydraulic wedge assembly having a wedge longitudinally displaceable against a complementary ramp. The ramp engages and supports the shoe and is constrained to move in a direction towards or away from the wheel. By mounting such a support mechanism at a tangent to the wheel so that shoe displacement is radial it is possible to control the gap size. However, a unidirectional active shoe positioning system is less than wholly satisfactory at least in part because of difficulties in adapting different shoe types used for radial and tangential mode extrusion and because it is desired to control the shape of the gap in addition to its size. To completely control both the size and the shape of the gap, as independent variables, it is preferred to provide the support mechanism with a first and a second wedge assembly. The first wedge assembly is disposed to displace the shoe in a first direction perpendicular to the axis of rotation of the wheel and the second wedge assembly is disposed to displace the shoe in a direction perpendicular to the rotary axis of the wheel and the first wedge assembly. The directions will ordinarily be the vertical and horizontal.
It is preferred that each wedge assembly includes an hydraulic ram to longitudinally displace the wedge.
Although wedges, ramps and rams are thought to be the best way of implementing the support mechanism at this time it is conceived that the use of hydraulic rams alone or ball screw driven rams may be capable of providing a support mechanism.
Means such as Poly-Tetra-Fluoro-Ethylene (PTFE) surfaces may be provided to reduce the friction between the wedge and the wedge bearing.
Preferably, where two wedge assemblies are provided to implement a bi-directional dynamic or active shoe positioning process, it is preferred to provide a gap sensor system having three gap sensors each located peripherally spaced from the other, to sense the size and shape of the gap.
An alternative arrangement would be for the shoe to be supported in the chassis by means of a pivot and swung into position to set the gap size. By supporting the pivot to be displaceable radially via the operation of a first actuator such as an hydraulic ram, and arranging for a second actuator such as a second hydraulic ram to be capable of swinging the shoe around the pivot, the size and shape of the gap may be dynamically adjusted during machine operation in accordance with the size and shape of the gap sensed by the gap sensor.
In order to sense both the size and shape of the gap the gap sensor system will preferably comprise a plurality of gap sensors deployed to detect the gap size at positions spaced circumferentially around the wheel.
Preferably the gap sensor system comprises gap sensors which sense the gap size directly to avoid the corrections required if the gap size and shape is sensed indirectly. To this end each gap sensor must tolerate the hostile environment at the interface between the wheel and the shoe while continuing to measure with accuracy of the order of 0.1 mm, so that a gap size of 0.2 mm can be accurately set. The sensor range will preferably exceed 0.5 mm to facilitate starting the machine and ideally will exceed 1 mm. The preferred form of sensor is a sonic gap sensor.
A sonic gap sensor relies on the principle that fluid flow through an orifice will choke when a fluid pressure upstream of the orifice reaches a critical pressure at which the flow through the orifice is sonic. In this condition the fluid condition downstream of the orifice has no influence on the conditions upstream of the orifice. When the orifice is choked the fluid condition upstream of the orifice correlates with the size of the orifice. By making the gap the orifice the size of the gap can be measured. Thus the gap . sensor of the present invention consists of at least one port located in the shoe adjacent the gap and a gas delivery pipe for delivering compressed gas to the port at or above the critical pressure. Pressure sensitive transducers are deployed in the gas delivery pipe in order to sense the gas pressure in the pipe. Once calibrated, changes in the gas pressures sensed can be used to determine the size of the gap adjacent the port. Thus the gap size can be determined by coupling the pressure transducers to a computer or other dedicated processor of the control means.
Sensors other than sonic gap sensors as presently available cannot tolerate the environment in the gap for sufficient time to be practical in a production machine. Improvements in the environmental tolerance of such sensors or even completely new types of sensor would obviously require reconsideration of the applicability of the sensor to this invention for directly sensing the gap size.
Indirect sensing of the gap size,, (ie. computation of the gap size from remote measurements) has been contemplated because this avoids many of the difficulties inherent in locating a sensor in the hostile environment within the gap. Sensors considered potentially suitable for indirect sensing include eddy current sensors, proximity sensors, optical sensors and hall effect sensors. Systems in which the gap size is sensed indirectly are considered to be within the broadest scope of this invention. Sensors from the previously mentioned list may be used to sense the gap by sensing the relative positions of the shoe, the wheel and possibly the chassis. Such a system will require the data from the indirect sensor(s) to be corrected for thermal and mechanical strain on the, wheel shoe and chassis. While not impossible the difficulties of correction are believed to be more disadvantageous than the difficulties of directly sensing the gap size.
The majority of material in the gap is confined to the areas of the wheel adjacent to the groove. When using gap sensors wheels 50 mm wider than is conventional are used and the sensors operate at the outer 25 mm which is clear of the flash. It is preferred to locate one gap sensor adjacent the mouth of the start of the tooling, one at the centre of the tooling and one immediately downstream of the abutment. So that the gap is the only significant constriction in the gap sensor, each port has a diameter approximately four times the maximum size of the gap. Preferably each gap sensor will comprise one port, overlying the edge of the wheel and communicating with an elongate gas delivery pipe. The gas port pressure (P) may be measured slightly upstream of the ports (e.g. at about 0.05 m) and a delivery pressure (P.) far upstream of the ports (e.g. at about 0.750 m). The ratio of the port pressure to the delivery pressure is approximately proportional to the size of the gap.
Conventionally a scraper is required to remove excess flash from the wheel rim during machine operation in order to prevent the flash build up from fouling the gap as it re-enters the shoe. However, problems arise in setting the scraper position relative to the wheel because of thermal expansion and blade tip wear during machine operation which alters the relative position of the scraper blade and the wheel. To alleviate this there may be provided a scraper carrier supported for radial displacement toward and away from the wheel rim and supporting a scraper blade at its tip adjacent the wheel rim. The scraper carrier is rendered radially displaceable during machine operation by a device such as an eccentric shaft, and a motor arranged to rotate the shaft to a degree determined by the control device. The control device responds to a gap sensor mounted on the tip of the scraper carrier to determine the separation the scraper blade tip and the wheel rim.
According to another aspect of the present invention there is provided a method of operating a continuous extrusion machine wherein feedstock is entrained in a groove formed in the periphery of a wheel rotating in a chassis and drawn into a passage formed between the groove and a shoe, said passage being obstructed by an abutment supported by the shoe so that friction between the shoe and the abutment will cause the feedstock to extrude through a die supported in the shoe, comprising the steps of: sensing the actual size of a gap between the wheel and the shoe, comparing the actual size of the gap with a predetermined or previous gap size in a control means to determine if there is a difference, said control means responding to a difference to control a support structure which supports the shoe and/or the wheel in the chassis to displace the shoe and/or the wheel on at least one axis perpendicular to the axis of rotation of the wheel so that the gap is changed to reduce the difference.
The sensing of the gap size and the adjustment of the shoe position take place during operation of the machine. This may include the start up operation of the machine before extrusion has begun. As the machine warms up from a cold start, the gap size may be sensed continuously but is preferably sensed at intervals. When the gap size differs from a previous value, or possibly when it diverges from a predetermined value, a control device of the control means responds to adjust the support structure so that the shoe is moved relative to the wheel to bring the gap size back towards the desired size.
The desired gap size may be altered during machine operation. Thus while the method contemplates setting the gap size to that required for extrusion, and preventing significant deviation during extrusion, it also contemplates setting the gap size to one predetermined value during machine start up, altering that predetermined value during continuous extrusion and possibly further altering the value during shut down of the machine.
The method of sensing the gap size preferably comprises blowing air or another pressurised gas such as an inert gas through the gap at at least one, and possibly two or preferably three circumferentially spaced points adjacent the passage. The pressure with which the air is blown is sufficient to ensure that the gap is choked and the pressure in a delivery pipe upstream of the gap can then be sensed and correlated with the gap size. It is preferred to sense the pressure and hence the gap at intervals in order to minimise the gas requirement.
The method may also comprise the steps of sensing the shape of the gap, in particular by sensing the size of the gap at two or more peripherally spaced locations and the step of adjusting the shape of the gap to a desired shape.